1. Field of the Invention
This invention relates to a method of cleaning an apparatus for forming deposited film by plasma CVD process, a thermal CVD process or the like, which are hereinafter generically referred to as "chemical vapor deposition" (CVD) process.
2. Description of the Prior Art
Techniques for forming a functional film on a substrate according to the CVD process have been already widely used, for example, for uniformly forming film of photoconductive member on a drum in electrophotography. When a film is formed by the CVD process, a part of the reaction product is inevitably attached to surfaces other than the substrate to be coated, for example, the surfaces of the inside walls of the reaction chamber, etc. in the form of film or powder. Such film and power attached to the inside walls will be easily peeled or dropped off. The powder dropped off floats and travels in the reaction chamber and then may adhere to the surface of the substrate to be coated with the deposited film, thereby causing defects of the film such as pin-holes. In order to prevent such defects, each time film formation is completed, it is necessary to demount the reaction chamber from the apparatus and then to remove the film and the powder attached to the inside walls by a physical or chemical method, for example, by immersing the reaction chamber in a solution containing hydrofluoric acid or the like to dissolve the attached substances. Then the reaction chamber is washed with pure water and the like and dried. Thereafter the reaction chamber is mounted on the apparatus again. The apparatus must be checked for air tightness and electrical contact before use. A series of procedures described above is extremely complicated. This causes lot to lot variation of the quality of the deposited film formed on the substrate, especially in the case of electrophotographic photosensitive member of amorphous silicon (which is, hereinafter, referred to as "a-Si" ).
It is difficult to automate the above washing step because the whole step must be inevitably conducted manually by operators. The larger apparatus for mass production is associated with larger equipment required for the washing step. Such drawbacks make difficult mass production of the film a-Si electrophotographic photosensitive members.
There has been known in the prior art a method of cleaning the reaction chamber by etching the inside walls thereof by use of nitrogen fluoride or ammonium fluoride as an etchant. However, in this case, silicon nitride (Si.sub.3 N.sub.4) forms on the surface of the inside walls in the course of etching step and covers the surface to be etched to reduce the etching rate.
There has been also known a method using a chloric compound as an etchant. In this case, impurities such as chlorine, etc. may be entrained in the silicic compound film formed in the reaction chamber after cleaning to considerably lower the film characteristics.
According to another method, hydrogen (H.sub.2) containing carbon tetrafluoride is used as an etchant. In this case, a polymer of CF.sub.2 monomer is produced and deposited on the surface to be etched. The deposition rate of the polymer may exceed the etching rate to spoil the etching effect.